Heterogeneous response of hydrogeological systems to the Izmit and Duezce (Turkey) earthquakes of 1999

At four sites in Turkey and Armenia the physico-chemical properties of thermal and mineral waters were monitored continuously during the Izmit and Duezce earthquakes that occurred along the North Anatolian fault in August and November 1999. The epicentral distances between the moment magnitude (M sub(w)) 7.6 Izmit earthquake and the monitoring locations were 313, 488, 1,161, and 1,395 km. At the most distant site, the specific electrical conductivity of mineral water from a flowing artesian well dropped co-seismically and postseismically by 7%. No changes were observed at the other sites, although the estimated earthquake strains and peak ground accelerations are much higher. A similar pattern was observed after the Duezce earthquake, which happened three months after the Izmit event. The response of a hydrogeological system seems to depend on the site characteristics rather than on the nature of the earthquake. A hydrogeological model for the sensitive observation site farthest from the Izmit earthquake explains the observations in terms of a changed mixing ratio between two fluid components. Passing seismic waves may trigger a local pore- pressure increase according to the mechanism of advective overpressure. The preconditions for this mechanism, free gas bubbles in the aquifer in combination with a trap for rising bubbles, is probably not fulfilled by the other groundwater systems.Original Abstract: Les caracteristiques physico-chimiques d'eaux thermo-minerales de quatre sites de Turquie et d'Armenie ont ete suivies en continu au cours des seismes d'Izmit et de Duezce, qui se sont produits le long de la faille nord-anatolienne en aout et novembre 1999. Les distances a l'epicentre entre le seisme d'Izmit de magnitude (Mw) 7,6 et les sites suivis etaient de 313, 488, 1.161, et 1.395 km. Pour le site le plus eloigne, la conductivite electrique specifique d'une eau minerale provenant d'un puits artesien jaillissant a baisse de 7% a la suite du seisme. Aucun changement n'a ete observe aux autres sites, bien que les contraintes estimees du seisme et les accelerations du sol au moment du pic aient ete plus importantes. Les memes effets ont ete observes a la suite du seisme de Duezce, qui s'est produit 3 mois apres celui d'Izmit. La reponse du systeme hydrogeologique parait dependre des caracteristiques du site plutot que de la nature du seisme. Un modele hydrogeologique pour le site d'observation sensible le plus eloigne du seisme d'Izmit explique les observat